Low density plasmas have become convenient sources of energetic ions and activated atoms which can be employed in a variety of semiconductor workpiece fabrication processes including surface treatments, depositions, and etching processes. (Semiconductor workpieces can include semiconductor wafers as well as glass compositions having semiconductor materials deposited thereon.) For example, to deposit materials onto a semiconductor wafer using a sputter deposition process, a plasma is produced in the vicinity of a sputter target material which is negatively biased. Ions created adjacent to the target impact its surface to dislodge, i.e., “sputter,” material from the target. The sputtered materials are then transported and deposited on the surface of the semiconductor wafer.
Sputtered material has a tendency to travel in straight line paths from the target to the substrate, at angles which are oblique to the surface of the substrate. As a consequence, materials deposited in etched trenches and holes of semiconductor devices having trenches or holes with a high depth to width aspect ratio, can bridge over causing undesirable cavities in the deposition layer.
To prevent such cavities, the sputtered material can be redirected into substantially vertical paths between the target and the substrate by negatively charging the substrate to position vertically oriented electric fields adjacent the substrate if the sputtered material is sufficiently ionized by the plasma. However in systems which employ balanced magnetrons, material sputtered in a low density plasma often has an ionization degree of less than about 5% which is usually insufficient to avoid the formation of an excessive number of cavities. (In systems which employ unbalanced magnetrons, the ionization degree can be about 20%.) Accordingly, it is desirable to increase the density of the plasma to increase the ionization rate of the sputtered material in order to decrease the formation of unwanted cavities in the deposition layer. As used herein, the term “dense plasma” is intended to refer to one that has a high electron and ion density.
Frequently, magnets or a magnetron are positioned in back of the target to create a magnetic field adjacent to the target. The magnetic field traps electrons in the plasma region thus increasing the ion density. As a result, the plasma density and sputter rate are increased.
However advances in semiconductor processing designs have resulted in the need for increasingly dense plasmas. This in turn has resulted in the use of increasingly strong magnets and magnetrons positioned in the back of targets. Such magnets often produce stronger magnetic fields which can extend well beyond the plasma chamber. Sometimes, such fields can extend to and interfere with adjacent equipment such as other plasma chambers or other equipment.
The present invention is directed toward improvements in the designs of semiconductor fabrication devices for reducing the magnetic fields external to the source plasma chamber.